Christian here (and very long-time lurker), of more or less the "I believe on faith" stripe. I've noticed that in addition to the uses you mention here, /Mere Christianity/ serves as a summary of doctrine. (I can't really speak to other apologetics.)
I've found that Sunday school and sermons generally incline much more to vague, feel-good platitudes than to actual, you know, positive or normative claims. You're left in this position of "OK, I have [for whatever reason] bought into the Church and its credibility---now what do these people actually believe? How can I draw on the collective experience of all these people?" /Mere Christianity/ tells you what, exactly, the vast majority of those people you find credible believe.
Further, it does so in a way that attempts to make those things plausible and memorable, even though you believe them not because of the Lewis' attempts at plausibility, but because of the credibility of the people who've given their assent to those things. This is much more readable, enjoyable, and amenable to discussion than a catechism. It's sort of like worked-out examples or concrete special cases in a textbook.